A method for early detection of undesired microbial infection in a cell culture by means of an electronic nose having an array of gas sensors and the ability of pattern recognition is described, said method comprising the steps of withdrawing an off-gas sample containing volatile compounds from the cell culture in a bioreactor, passing said sample to the gas sensors of the electronic nose, and comparing any specific gas sensor signal patterns with standard signal patterns corresponding to different micro-organisms, wherein the off-gas sample is withdrawn from an animal cell culture, and the detection of any microbial infection is established about 1-50 h earlier for each micro-organism compared to known techniques.